Due to the amorphous (chemical) physical structure, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals.

It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.

At microscopic scales, precious opal is composed of silica spheres some 150–300 nm (5.9×10−6–1.18×10−5 in) in diameter in a hexagonal or cubic close-packed lattice.

It was shown by J. V. Sanders in the mid-1960s[8][9] that these ordered silica spheres produce the internal colors by causing the interference and diffraction of light passing through the microstructure of the opal.

[11] In addition, microfractures may be filled with secondary silica and form thin lamellae inside the opal during its formation.

In gemology, opalescence is applied to the hazy-milky-turbid sheen of common or potch opal which does not show a play of color.

The darker backing emphasizes the play of color and results in a more attractive display than a lighter potch.

An opal triplet is similar to a doublet but has a third layer, a domed cap of clear quartz or plastic on the top.

Their cell walls are made up of hydrated silicon dioxide which gives them structural coloration and therefore the appearance of tiny opals when viewed under a microscope.

[18] Diatomite has multiple industrial uses such as filtering or adsorbing since it has a fine particle size and very porous nature, and gardening to increase water absorption.

The precious opal is hosted and found in situ within a subsurface horizon or zone of bentonite, which is considered a "lode" deposit.

Although fire opals don't usually show any play of color, they occasionally exhibit bright green flashes.

[15]Opal occurs in significant quantity and variety in central Mexico, where mining and production first originated in the state of Querétaro.

In this region the opal deposits are located mainly in the mountain ranges of three municipalities: Colón, Tequisquiapan, and Ezequiel Montes.

In 1957, Alfonso Ramirez (of Querétaro) accidentally discovered the first opal mine in Jalisco: La Unica, located on the outer area of the volcano of Tequila, near the Huitzicilapan farm in Magdalena.

There are also some small opal mines in Morelos, Durango, Chihuahua, Baja California, Guanajuato, Puebla, Michoacán, and Estado de México.

Other significant deposits of precious opal around the world can be found in the Czech Republic, Canada, Slovakia, Hungary, Turkey, Indonesia, Brazil (in Pedro II, Piauí[50]), Honduras (more precisely in Erandique), Guatemala, and Nicaragua.

[10] The resulting material is distinguishable from natural opal by its regularity; under magnification, the patches of color are seen to be arranged in a "lizard skin" or "chicken wire" pattern.

While many genuine opals are cut and polished without a matrix, the presence of irregularities in their play-of-color continues to mark them as distinct from even the best lab-created synthetics.

Other research in macroporous structures have yielded highly ordered materials that have similar optical properties to opals and have been used in cosmetics.

[57][58] The lattice of spheres of opal that cause interference with light is several hundred times larger than the fundamental structure of crystalline silica.

[59] Dehydration experiments and infrared spectroscopy have shown that most of the H2O in the formula of SiO2·nH2O of opals is present in the familiar form of clusters of molecular water.

Isolated water molecules, and silanols, structures such as SiOH, generally form a lesser proportion of the total and can reside near the surface or in defects inside the opal.

The structure of low-pressure polymorphs of anhydrous silica consists of frameworks of fully corner bonded tetrahedra of SiO4.

Microcrystalline opal or Opal-CT has been interpreted as consisting of clusters of stacked cristobalite and tridymite over very short length scales.

[60] Opal-CT, also called lussatine or lussatite, is interpreted as consisting of localized order of α-cristobalite with a lot of stacking disorder.

Noncrystalline silica in siliceous sediments is reported to gradually transform to opal-CT and then opal-C as a result of diagenesis, due to the increasing overburden pressure in sedimentary rocks, as some of the stacking disorder is removed.

[5] As references to the gem are made by Pliny the Elder, one theory attributed the name's origin to Roman mythology: to have been adapted from Ops, the wife of Saturn, and goddess of fertility.

However, historians have noted the first appearances of opallios do not occur until after the Romans had taken over the Greek states in 180 BC and they had previously used the term paederos.

When a drop of holy water falls on the talisman, the opal turns into a colorless stone and the Baroness dies soon thereafter.